Plant and method for dry extracting/cooling heavy ashes and for controlling the combustion of high unburnt content residues

ABSTRACT

The present invention relates to a system for dry extracting/cooling heavy ashes and for controlling the combustion of high unburnt content residues, allowing to: extract heavy ash from the boiler bottom ( 12 ), foster and adjust post-combustion on the extractor belt ( 14 ) by combined use of comburent hot air and inert combustion fumes, already available in boiler, cool the ashes present on the belt and optionally re-circulating them—all or in part—in boiler, along with the fraction of light ashes of higher unburnt content.

FIELD OF THE INVENTION

The present invention refers to a plant and a method for dry, or mainlydry extracting/cooling and reducing unburnts in heavy ashes produced byboilers of the type used in solid fuel thermoelectric power plants.

Such plant and method are specifically suitable in the case of boilersburning, under co-combustion, conventional solid fuel (typically coaldust) and non-conventional fuel, especially biomasses and/or fuelderived from municipal solid waste (RDF).

BACKGROUND OF THE INVENTION

The need to reduce CO₂ emissions urged to use alternative fuels in lieuof coal, such as biomasses and the so-called “RDF” (fuel derived frommunicipal solid waste).

If on the one hand the use of biomasses, and of non-conventional fuelsin general under co-combustion with coal dust, allows a reduction intotal CO₂ emissions in the atmosphere, on the other it entails a seriesof problems related to the combustion system, among which that of fuelpulverizing. In fact, though being very reactive with respect to coal,such alternative fuels, and particularly biomasses, require remarkableenergy amounts in order to be crushed to an appropriate degree/level,thereby ensuring high combustion efficiency. Moreover, an excessivelevel of crushing causes a greater wear of the grinding members.

Therefore, in common practice, both in order to limit energy consumptionand increase the life of said wearing parts of the grinding members, itis preferred to crush biomasses to a coarser grade, thereby reducing,however, the combustion efficiency.

Accordingly, a merely partial combustion of the coarser biomassparticles causes an increase in the amount of unburnts in heavy andlight ashes.

Said heavy ashes are removed from the bottom of the combustion chamberby a dry extracting/cooling system typically made as illustrated inEuropean Patent EP 0 471 055 B1, and may undergo an at least partialpost-combustion on such an extracting system, so as to reduce unburntcontent in end ashes.

However, in known-art plants the management modes of such apost-combustion, above all in the most critical applications associatedto the use of the above-illustrated biomasses and RDF, are less thanoptimal and allow a merely limited reduction of total unburnts. Inparticular, in known plants there is anyhow the risk of uncontrolledpost-combustion phenomena on the extractor; said risk limits, alsoimplicitly, the (full) usability of measures for triggering or fosteringsuch a post-combustion.

SUMMARY OF THE INVENTION

Hence, on the basis of what has been disclosed in the preceding section,the technical problem set and solved by the present invention is toprovide a plant and a method for the post-combustion of unburnts in asystem for dry—or mainly dry—extracting heavy ashes, overcoming thedrawbacks mentioned above with reference to the known art.

Such a problem is solved by a plant according to claim 1 and a methodaccording to claim 30.

Preferred features of the present invention reside in the dependentclaims thereof.

The present invention provides several relevant advantages. The mainadvantage lies in that it allows, above all in the mentioned case ofbiomass or RDF co-combustion, an effective and efficient post-combustionof unburnts on the primary extractor, thereby reducing the total contentthereof, concomitantly allowing to avert the risk of an uncontrolledpost-combustion.

In particular, to foster unburnt reduction the invention preferably actsboth on the temperature of the heavy ashes extracted and on theirresidence time in an environment with a suitable temperature, typicallythe belt-equipped extractor portion arranged immediately downstream ofthe combustion chamber and facing thereon. With the increase of thetemperature in the post-combustion zone and of the related residencetime of the fuel therein, the combustibility rate of the latterincreases proportionally. To increase the temperature, the inventionprovides the inletting of hot air into the extractor and, in a preferredembodiment thereof, fuel residence time is adjusted by acting on thespeed of the conveyor belt. According to the invention, to control thecombustion process and prevent an excessive amount of biomass or RDFfrom leading to uncontrolled development of heat, combustion exhaustgases (fumes)—preferably collected downstream of the electrofilter (orelectrostatic precipitator) typically provided in plants to which theinvention applies—are used to partially or completely replace comburentair.

Hence, the invention allows to increase the post-combustion capacity ofdry or mainly dry extractors, making the extracting environment whereashes are moved more favorable to the reduction of unburnts present inthe ashes themselves.

The combined use of hot air and combustion fumes according to theinvention allows to have total control on unburnt combustion on theextractor-afterburner belt.

As mentioned above, the post-combustion process preferably occurs and iscompleted in the extractor zone corresponding to the throat at theboiler bottom and optionally, if necessary and advisable for plantrequirements, also in the subsequent section thereof.

Moreover, in a preferred embodiment it is provided, downstream of asuitable cooling of the extracted heavy ashes, the in-boilerre-circulating thereof, along with the fraction of light ashes of higherunburnt content.

Always on the basis of a preferred embodiment, downstream of thepost-combustion process there starts the cooling of the ash by air,which, in a controlled and adjusted amount, is let into the extractor,in the end portion of conveying, and/or in a secondary conveyor/cooler(post-cooler) arranged downstream of the primary extractor. Preferably,at the outlet of the conveyor-cooler an ash crushing stage (step) isprovided, and, downstream thereof, a screening station allowing toeliminate any plastics or metallic residue present in the ashes. Thus,it is possible to obtain ash suitable to be stored or optionallyre-circulated in a boiler.

Preferably, the permanence time of light ashes in said electrofilter (orelectrostatic precipitator) is reduced by continuously re-circulating incombustion chamber the unburnt-richer fraction of light ashes. Thus, theinvention allows to reduce the risks of fires in the electrofilters whenas fuels there are utilized biomasses or RDF that, by accumulating inthe hoppers of the electrofilters themselves can cause fires byself-combustion, biomass or RDF unburnts being very reactive withrespect to those from coal.

Total unburnt reduction, attained both through post-combustion on theconveyor belt and re-circulating in combustion chamber, allows to saveon consumption of ammonia utilized for NO_(x) reduction in catalyzers.In fact, to attain the same unburnt content in the total ashes, in theabsence of re-circulation, a higher excess of combustion air would berequired, with the entailed increase of NO_(x) rate in exhaust gases andof the amount of ammonia required for their reduction.

Summing up the detailed description of preferred embodiments reportedhereinafter, the present invention relates to a system allowing to:extract heavy ash from the boiler bottom, foster and adjustpost-combustion on the extractor belt by combined use of comburent hotair and inert combustion fumes, already available in boiler, cool theashes present on the belt and optionally re-circulate them—all or inpart—in boiler, along with the fraction of light ashes of higher unburntcontent.

BRIEF DESCRIPTION OF THE FIGURES

Other advantages, features and the operation modes of the presentinvention will be made evident from the following detailed descriptionof some preferred embodiments thereof, given by way of example andwithout limitative purposes. Reference will be made to the figures ofthe annexed drawings, wherein:

FIG. 1 shows a general exemplary diagram of a preferred embodiment ofthe plant of the invention;

FIG. 2 shows a cross section of an extractor—afterburner of the plant ofFIG. 1, taken along line A-A of the latter figure; and

FIG. 3 depicts a top plan view of a detail of a drilled extractor beltof the plant of FIG. 1.

DETAILED DESCRIPTION

Referring to said figures, a combustion plant made according to apreferred embodiment of the invention is generally denoted by 1.

The plant 1 is of the type used in thermoelectric solid fuel powerplants firing solid fuel, in particular coal dust, and it is suitablefor the (co-)combustion of biomasses and/or fuel derived from municipalsolid waste (RDF).

In the present example, the plant 1 will be described with reference tothe co-combustion of biomasses.

For clarity's sake, the various components of the plant 1 willhereinafter be described mainly with reference to the path followed bybiomasses, starting from their collection from storing means down totheir combustion, and with reference to the path followed by combustionresidues (heavy and light ashes) starting from their collection from thebottom of a combustion chamber (or boiler) 12 of the plant 1 down totheir post-combustion, optional re-circulating in the combustion chamberitself and discharge.

First of all, the plant 1 provides as biomass storing means the samebunkers, per se known, already used for coal. In the present embodiment,the biomass-dedicated bunkers are the two depicted on the left in FIG. 1and denoted by 21 and 22, respectively. The other bunkers and theassociated additional components depicted in FIG. 1 are understood asused for coal, and therefore will not be further considered hereinafter,their structure and use being of a per se known type.

Preferably, the bunkers 21 and 22 are those feeding the burners of thecombustion chamber 12 at topmost levels, so as to have for the heavierparticles a longer residence time in the combustion chamber itselfduring their falling to the bottom.

From the dedicated bunkers 21 and 22, the biomass is extracted by one ormore conveyors 3 analogous to those already used for coal, or by augers.Thus, biomass is supplied, by an intercepting valve 4, to a dedicatedmeter 5, in this case with three outlets, respectively denoted by 51, 52and 53, and therefrom to one or more dedicated crushers, in this casethree, respectively denoted by 61, 62 and 63. In the present embodimentsaid crushers 61-63 are implemented by per se known hammer mills.

Hence, said conveyors 3 constitute bypass means of known coal crushers,here denoted by F, associated to the bunkers 21 and 22. Such a bypasstherefore allows the plant 1 not to overly modify its own standardconfiguration with respect to that related to the combustion of coalalone, allowing to leave installed also said known crushers F.

The crushers 61-63 are apt to reduce the biomass to a desired maximumend (outlet) grain size. As it will be better appreciated at the end ofthe description, it is not required that said final grain size beparticularly fine, since the overall structure of the integrated plant 1allows anyhow a complete combustion of the biomass even at such “coarse”grain sizes.

Downstream of the mills 61-63 there are respective screening means 71,72 and 73 apt to intercept biomass particles of a grain size greaterthan a predetermined threshold, in order to resend them, throughdedicated mechanical or pneumatic conveyors 8, to the meter 5 and theninto the same hammer mills 61-63 for a new crushing thereof.

Finer biomass particles crossing the screening means 71-73 are carriedby a common (shared) conveyor 9 to a single meter 10 and then, by meansof a two-way valve 91, fed to known-type pneumatic conveyors 93, thelatter already present in existing solid fuel plants in association withthe crushers F. I.e., the pneumatic conveyors 93 are those developingfrom the coal crushing mills F (not used) associated to the bunkers 21and 22.

Then, crushed biomasses are introduced in the feed pipes feeding coaldust, and therefrom fed to the burners of the boiler 12, it also of atype already present in existing solid-fuel plants.

Once the fuel comprised of said coal and biomasses is fed to thecombustion chamber 12, the plant 1 and the extraction andpost-combustion process carried out therefrom develop as describedhereinafter.

Fly ashes leave the combustion chamber 12 through traditional ducts(flues) for expelling combustion fumes, generally denoted by 13 inFIG. 1. Instead, the heavy portion of ashes and of any unburntprecipitates bottomwise and is collected on a conveyor-type dryextractor 14, of the kind subject-matter of EP 0 471 055 and not furtherdescribed herein.

According to the invention, on said extractor 14, and in particular on apost-combustion or afterburning portion 141 thereof facing onto thecombustion chamber 12, unburnt post-combustion goes on. To this end, thetop surface of the extractor belt, and in particular of the portion 141,receives heat by irradiation from the burners of the combustion chamber12.

According to a variant embodiment, the plant of the invention may alsocomprise a biomass meter 18 independent of the combustion chamber 12 anda feeder 19, depicted also in FIG. 1, arranged upstream of the extractor14 (or at least upstream or in correspondence of the portion 141thereof), to supply unburnt biomass directly to the extractor 14 itself,for a first combustion of the latter biomass in the post-combustion zone141. In this case, a first drying of said biomasses may be carried outby a feeding of hot air on the feeder 19; advantageously, said hot airmay come from an air/fumes exchanger 29, already present in existingthermoelectric plants and that will be introduced hereinafter.

According to the invention, to allow a higher post-combustion efficiencyand concomitantly avoid the development of uncontrolled phenomena, it isprovided control means 100 for controlling unburnt post-combustionoccurring on said post-combustion portion 141.

Said means 100 in turn comprises means 15 for feeding hot air and means150 for feeding combustion exhaust gases (fumes), apt to provide a flowof heated air and of combustion fumes, respectively, in correspondenceof said post-combustion portion 141 in order to respectively foster andinhibit post-combustion.

In the present embodiment, the feeding means 15 and 150 comprisesrespective ducts for the collecting respectively of heated air from anair chamber 151 associated to the boiler 12 and of exhaust fumesdownstream of an electrostatic precipitator (electrofilter) 28 of theplant 1.

Typically, hot air in the air chamber 151 comes from the above-mentionedexchanger 29, which in the present embodiment is a fumes/air exchangerarranged downstream of the combustion chamber 12 and exploiting just theresidual heat of the combustion fumes to heat outside air. According toa variant embodiment, hot air fed by the means 15 may also be bleddirectly from the latter exchanger 29.

Said air chamber 151, air pre-heater 29 and electrostatic precipitator28 are well-known to a person skilled in the art and already present inknown plants; therefore, a further description thereof will be omitted.

The means 15 for feeding hot air comprises means 143 for the controlled(adjusted) inletting of outside air, e.g. inlets made on the casing ofthe extractor 14 and associated to one or more valves preferablycontrolled by the control means 100, allowing to attain a desired oxygencontent and an appropriate temperature of the air inlet into theextractor 14.

Hence, heated air fed by the means 15 and suitably proportioned withatmospheric air allows to attain on the extractor/afterburner belt 14 atemperature optimal for post-combustion.

Said means 143 can also exploit the negative pressure present in thecombustion chamber 12 for the feeding of air from the outside.

Preferably, the overall arrangement is such that the hot-air feedingmeans 15 and the fume feeding means 150 is apt to supply a flowcountercurrent with respect to the direction of advance of the belt ofthe extractor 14 itself, at least in the above-introducedpost-combustion portion 141.

Both the air feeding means 15 and the fume feeding means 150 may beequipped with respective means for automatically adjusting the flowrate, respectively denoted by 102 and 103 in FIG. 1, controlled by thecontrol means 100.

The control means 100 is apt to adjust the flow rate of hot air and/orof fumes fed into the post-combustion portion 141, and for this purposeit comprises automatic means for adjusting said flow rates of air andfumes depending on the temperature detected by suitable sensors,preferably arranged in correspondence of said post-combustion portion141. When the temperature value exceeds a preset threshold, fume flowrate is increased and accordingly hot air flow rate is reduced: Thus,oxygen concentration is reduced, reducing the combustion rate. Inparticular, the fumes produced by the combustion process of the boilermay integrate or replace comburent air to adjust or stop on-beltcombustion thanks to the low (<6%) O₂ concentration in the fumes,enabling its use as inert gas. Vice versa, when the temperature is lowerthan a preset limit, fume inletting is inhibited and hot-air flow rateis increased, optionally reducing also the flow rate of room-temperatureair introduced by the means 143.

For moving the fumes an additional fan may be installed, when required,to provide them with the head required for the inletting into theextractor/afterburner 14.

In order to avoid acid condensation problems, the piping for the fumesof the feeding means 150 should be insulated, just to hold a temperaturehigher than the condensation one.

The fumes utilized for controlling the combustion, in addition to theextinguishing power exhibit also an appreciable cooling capacity, sincetheir temperature is no higher than 150° C. In fact, in the presentembodiment said fumes come from the zone downstream of theelectrofilter, i.e. are collected when they have already lost theirthermal content.

Hence, according to a variant embodiment, the post-combustion portion141 is concentrated exclusively or nearly exclusively in the irradiatedzone below the throat of the boiler 12, and controlled by feeding hotair and/or fumes in the manner disclosed above. The extractor beltportion not facing onto the boiler bottom is instead dedicated to thecooling, which may be carried out by the feeding of combustion fumes(with dedicated means) and cold air (with the above-introduced means143), taking care to inlet the fluid in the conveying zone by exploitingthe negative pressure in the boiler, and so as to have the fluid lickthe cover of the extractor 14, cooling it.

Lastly, as a further option for controlling the combustion there may beprovided the use of water finely metered by means of delivery nozzles104 preferably provided in plural zones of the extractor/afterburner 14,and in particular (at least) in the primary crushing zone (i.e. in theend portion of the extractor 14 with respect to the direction of advanceof the ashes).

According to a preferred variant embodiment, hot air fed by the means 15is supplied below the belt of the extractor 14, and in particular belowthe portion 141 thereof, as mentioned above in countercurrent to theflow of heavy ashes and unburnts. In this case, in order to make heatexchange and post-combustion more effective and efficient, the belt ofthe extractor 14 may be provided with perforations (holes) or slots 142,shown in FIG. 3. Thus, hot air, in addition to heating the bottom of theextractor 14, crosses the bed of ash and unburnts and partially returnsinto the combustion chamber 12, thanks to the negative pressure presentin the latter, re-feeding heat therein. Such a passage of air isfostered by the pressure difference existing between the bottom portionof the belt conveyor and the bottom of the boiler. Hot-air transitthrough the holes of the belt conveyor of the extractor 14 allows agreater and more effective contact of air itself with the ash present onthe belt, with the result of enhancing the combustion efficiency of theunburnt material.

As already mentioned above, in the present embodiment onto the lateralwalls of the extractor 14- and in particular in correspondence of an endportion of the belt typically uninvolved in the post-combustion—it isprovided further air inletting means 143 for the controlled inletting ofoutside cooling air.

From the extractor 14, heavy ashes and unburnts are supplied to asecondary belt conveyor 16 serving as post-cooler, and this through aprimary crusher 20, preferably water-cooled to resist high temperatures,downstream of which it is positioned a transition hopper 201, merelysketched in FIG. 1.

On the cooling conveyor 16 an air-assisted cooling of the ashes iscarried out by a system in countercurrent, exploiting the negativepressure present in the combustion chamber to feed outside air viacontrolled inlets 160 present on the lateral walls of the casing of theconveyor 16 itself, as already described also in EP 0 471 055.

According to a preferred variant embodiment of the invention, saidhopper 201 forms part of a pressure insulation system apt to create justa pressure separation between the environments of the extractor 14 andof said cooling conveyor 16. For this purpose the hopper 201 forms meansfor accumulating the conveyed material, allowing the forming of a headof material between said environments, creating said pressureseparation.

Said pressure insulation system allows a more effective management ofthe air-assisted cooling of the ashes, as it allows—when required andtypically on the basis of ash temperature and flow rate detectionsperformed, e.g., at their discharge onto the extractor 14- to avoid theintroduction of an excessive amount of cooling air into the combustionchamber 12, just by selectively activating such a pressure separation,as described also in PCT/IT2006/000625.

The head of material forming at the level of the hopper 201 may beadjusted by acting on the relative and absolute advance speeds of theconveyors 14 and 16.

When the pressure insulation is activated, the heated air outlet fromthe conveyor 16 is fed into a section 26 of the fume ducts 13 of theplant 1 by a further suitable duct 25 originating from the end portionof the conveyor 16 itself and provided with means for automaticallyadjusting the flow rate. Such a connection between the post-cooler 16and the boiler side may occur upstream or downstream of theabovementioned air pre-heater 29 (fumes side) arranged along the fumeducts 13. Through said connection, cooling air inlet to the post-cooler16 is recalled by the negative pressure value existing in saidboiler-side zone.

Preferably, the plant 1 provides also a bypass piping or duct, (forsimplicity's sake omitted from the figures) connecting theextractor/afterburner 14 with the conveyor-cooler 16 and provided withautomatic opening/closing valve.

Downstream of the cooling conveyor 16 it is provided a secondary crusher202 of opposing roll or equivalent type, suitable for crushing ash only,and in particular capable of reducing the grain size of the latterwithout altering the grain size of any plastics material mixed to theash and deriving from RDF combustion. In this typology of crusher,already known to a person skilled in the art, plastics particles transitthrough the rolls deforming, yet without breaking.

Hence, it is provided a mechanical or pneumatic screening system 203located downstream of the second crushing stage (step) associated to thedevice 202, for separating ash from plastics particulate and anymetallic parts present in the RDF that are stored or forwarded todisposal by dedicated means.

Crushed ash is forwarded to coal pulverizing mills via a mechanical orpneumatic conveyor 204, so as to be re-circulated in combustion chamberthrough the coal pulverizing mills and the boiler burners as describedin the International Application PCT/EP2005/007536.

Concerning the path of the abovementioned combustion fumes generated inthe combustion chamber 12 and of the light ashes carried thereby, saidfumes cross the zone of the economizers 27 and are then inlet throughsaid fume ducts 13 in the hereto-mentioned light-ash electrostaticprecipitator 28 or equivalent means, optionally by crossing first thehereto-mentioned air/fumes exchanger 29.

The light ashes precipitated into said electrostatic precipitator 28 arecollected by suitable means 30 for their re-circulating in combustionchamber.

In short, therefore, after the post-combustion on the belt of theextractor 14 any unburnt still present in the heavy ash, after asuitable cooling by the conveyor 16; are re-circulated in combustionchamber along with the light ash having a higher unburnt content, toattain a full unburnt conversion.

Moreover, the plant 1 incorporates a central adjustment and controlsystem, capable of ensuring the automated carrying out of the stepsdescribed hereto.

Therefore, by now it will be appreciated that the described plant allowsto attain a reduction in unburnts present in heavy ashes, by acontrolled post-combustion process on the extractor belt and byre-circulating in boiler the heavy ashes thus produced and the lightashes having higher unburnt content, the latter ashes coming from theelectrofilters.

Moreover, it will be appreciated that the above-described integratedsystem for the dry extraction, crushing, biomass post-combustion andheavy ashes and unburnt re-circulating allows to enhance the combustioncapacity of biomasses (and generally of non-conventional fuel) and alsoto increase the combustion efficiency thereof.

The described system concomitantly allows the use of the apparatusesalready present in the existing thermoelectric power plants, reducingplant installation costs and requiring minimum adjustment measures ofthe existing ones. In particular, also biomasses (and, in general,non-conventional fuel masses) continue to be moved with the means usedfor moving coal and heavy ashes. Moreover, boiler bunkers currentlyrepresenting the means for accumulating traditional solid fuel materialcan be utilized for biomasses as well.

By now, it will also be appreciated that, unlike in the state of theart, in which it is provided the re-circulation in combustion chamber ofheavy ashes added to light ashes of higher unburnt content, using coalmills for their pulverizing, the present invention provides biomasscombustion directly on the extractor belt, utilizing part of thepre-heated air, and it optimizes mixed combustion of biomasses in coaldust boilers, by heavy ash re-circulating.

It will be appreciated that the invention also refers to a method forthe (co-) combustion of biomasses in a solid-fuel thermoelectric powerplant of the above-described type, said method providing the dryextraction of heavy ashes and unburnts from the combustion chamber 12 byan extractor 14 of the above-mentioned type, and wherein incorrespondence of the portion 141 of the latter there occurs apost-combustion of the unburnts controlled by the selective feeding of aflow of hot air and of a flow of combustion fumes in order torespectively foster and inhibit the post-combustion, the flow rate ofhot air and/or of fumes fed into said post-combustion portion beingadjusted.

Preferred features of said method have already been described withreference to plant 1.

Lastly, it will be understood that, even though the integrated systemdescribed hereto optimizes the combustion efficiency of biomasses (andof non-conventional fuel in general) and plant management, separateprotection might be required for the different aspects of the invention,and in particular for the system for controlling the combustion by airand fumes, for the dedicated crushing, for the biomass post-combustionsystem and for the means for the direct combustion of biomasses on theextractor by the dedicated meter, each of said aspects allowing howevera substantial improvement of said efficiency.

The present invention has been hereto described with reference topreferred embodiments thereof. It is understood that other embodimentsmight exist, all comprised within the protective scope of the claimshereinafter.

1. A combustion plant adapted to be used in a thermoelectric solid fuelpower plant in association with a combustion chamber for said fuel,comprising: a dry extractor of heavy ashes and unburnts from thecombustion chamber, adapted to be arranged downstream of the combustionchamber and comprising a post-combustion portion for the unburnts; andcontrol means for controlling unburnts post-combustion occurring on saidpost-combustion portion, the control means comprising means for feedinghot air and means for feeding combustion fumes, adapted to provide aflow of heated air and of combustion fumes, respectively, incorrespondence of said post-combustion portion in order to respectivelyfoster and inhibit post-combustion, said control means being adapted toselectively adjust the flow rate of hot air and/or fumes fed into saidpost-combustion portion.
 2. The plant according to claim 1, wherein saidpost-combustion portion is adapted to be arranged at least partiallyfacing the bottom of the combustion chamber so that the post-combustionexploits irradiation heat coming from the combustion chamber itself. 3.The plant according to claim 1, wherein said means for feeding hot airand/or means for feeding combustion fumes are configured to supply, incorrespondence of said post-combustion portion, an air flowcountercurrent with respect to a moving direction of said extractor. 4.The plant according to claim 1, wherein said control means comprisesautomatic means for adjusting said flow rates of air and/or fumesdepending on temperature detected in correspondence of saidpost-combustion portion.
 5. The plant according to claim 1, wherein saidmeans for feeding hot air and/or means for feeding combustion fumes areadapted to exploit, in order to foster air and/or fumes circulation,negative pressure present in the combustion chamber.
 6. The plantaccording to claim 1, wherein said control means comprises means forcontrolled inletting of outside air to be mixed to the hot air of saidfeeding means.
 7. The plant according to claim 1, wherein said means forfeeding hotair collects or is adapted to collect heated air from an airchamber or from a pre-heater.
 8. The plant according to claim 1, whereinsaid means for feeding combustion fumes collects or is adapted tocollect said fumes downstream of an electrostatic precipitator.
 9. Theplant according to claim 1, wherein the temperature of the combustionfumes fed by said means for feeding combustion fumes is equal to orlower than about 150° C.
 10. The plant according to claim 1, whereinsaid means for feeding combustion fumes comprises a fan for increasinghead of the flow.
 11. The plant according to claim 1, wherein said dryextractor has side inlets for controlled inletting of outside air. 12.The plant according to claim 1, wherein said dry extractor comprises anextractor belt comprising, at least in correspondence of saidpost-combustion portion, a plurality of perforations adapted to fosterpassage of air through the conveyed material.
 13. The plant according toclaim 1, comprising means for feeding cooling water in said extractor,arranged at least in correspondence of an end portion thereof.
 14. Theplant according to claim 1, comprising means for adjusting residencetime of unburnts in said post-combustion portion.
 15. The plantaccording to claim 14, wherein said means for adjusting is based oncontrol of speed of said extractor.
 16. The plant according to claim 1,comprising a cooling conveyor arranged downstream of said dry extractor.17. The plant according to claim 16, comprising pressure insulationmeans adapted to create a pressure separation between environments ofsaid extractor and said cooling conveyor.
 18. The plant according toclaim 17, wherein said pressure insulation means comprises means foraccumulating the conveyed material, adapted to allow forming of a headof material between said environments creating said pressure separation.19. The plant according to claim 1, comprising means for re-circulating,light ashes in the combustion chamber.
 20. The plant according to claim19, wherein said means for re-circulating comprises means for collectinglight ashes from or immediately downstream of an electrostaticprecipitator.
 21. The plant according to claim 1, comprising means forre-circulating, in the combustion chamber, unburnts contained in heavyash.
 22. The plant according to claim 1, comprising means for feedingsaid fumes into an ash cooling zone arranged downstream of saidpost-combustion portion of said extractor.
 23. The plant according toclaim 1, comprising an unburnt biomass feeder, arranged upstream or incorrespondence of said post-combustion portion of said extractor andindependent from the combustion chamber, said feeder being adapted tolay unburnt biomass directly on said extractor.
 24. The plant accordingto claim 23, comprising means for feeding hot air to said feeder,adapted to perform a first drying of the unburnt biomass.
 25. The plantaccording to claim 24, wherein said means for feeding hot air to thebiomass is associated with an air/combustion fumes exchanger.
 26. Theplant according to claim 1, comprising dedicated crushing means,arranged or adapted to be arranged upstream of the combustion chamberand suitable for crushing biomass according to a preset maximum outletgrain size.
 27. The plant according to claim 26, wherein said dedicatedcrushing means comprises one or more hammer mills.
 28. The plantaccording to claim 26, comprising bypass means adapted to supply thebiomass from storage bunkers to said dedicated crushing means.
 29. Theplant according to claim 26, comprising screening means arrangeddownstream of said dedicated crushing means and adapted to interceptbiomass particles of a grain size greater than a predetermined thresholdto resend said biomass particles to said crushing means.
 30. Acombustion method adapted to be used in a thermoelectric solid fuelpower plant, comprising a combustion chamber for said fuel, said methodcomprising: providing dry extraction of heavy ashes and unburnts fromthe combustion chamber by an extractor arranged downstream of thecombustion chamber, and performing post-combustion of the unburntsoccurring on a post-combustion portion of said extractor, saidperforming post-combustion being controlled by selective feeding of aflow of hot air and of a flow of combustion fumes in order torespectively foster and inhibit the post-combustion, wherein flow rateof hot air and/or of fumes fed into said post-combustion portion isadjusted.
 31. The method according to claim 30, wherein saidpost-combustion portion is arranged at least partially facing the bottomof the combustion chamber, so that the post-combustion exploitsirradiation heat coming from the combustion chamber itself.
 32. Themethod according to claim 30, wherein said feeding of hot air and/or offumes supplies, in correspondence of said post-combustion portion, aflow countercurrent with respect to direction of advancement ofcombustion residues.
 33. The method according to claim 30, wherein saidflow rates of air and/or fumes is automatically adjusted depending ontemperature detected in correspondence of said post-combustion portion.34. The method according to claim 30, wherein said feeding of hot airand/or of fumes exploits, in order to foster air and/or fumescirculation, negative pressure present in the combustion chamber. 35.The method according to claim 30, wherein a controlled feeding ofoutside air is provided into said post-combustion portion.
 36. Themethod according to claim 30, wherein said feeding of hot air comprisescollecting heated air from a pre-heater or from an air chamber.
 37. Themethod according to claim 30, wherein said feeding of combustion fumesprovides collecting of said fumes downstream of an electrostaticprecipitator.
 38. The method according to claim 30, wherein temperatureof the combustion fumes fed into said post-combustion portion is equalto or lower than about 150° C.
 39. The method according to claim 30,wherein said feeding of combustion fumes comprises using ventilationmeans for increasing head of the flow.
 40. The method according to claim30, further comprising providing controlled feeding of outside air intothe dry extractor.
 41. The method according to claim 30, furthercomprising feeding cooling water into said extractor, at least incorrespondence of an end portion thereof.
 42. The method according toclaim 30, further comprising adjusting residence time of unburnts insaid post-combustion zone.
 43. The method according to claim 42, whereinsaid adjusting is based on speed control of said extractor.
 44. Themethod according to claim 30, comprising a step of cooling downstream ofthe extracting carried out by said dry extractor.
 45. The methodaccording to claim 44, further comprising providing an option ofactivating a pressure insulation between environments of said extractorand said cooling.
 46. The method according to claim 45, wherein saidpressure insulation is activated by means for accumulating the conveyedmaterial, adapted to allow forming of a head of material between saidenvironments creating said pressure separation.
 47. The method accordingto claim 30, further comprising re-circulating light ashes in thecombustion chamber.
 48. The method according to claim 47, wherein thelight ashes to be re-circulated are collected from or immediatelydownstream of an electrostatic precipitator.
 49. The method according toclaim 30, further comprising re-circulating, in the combustion chamber,unburnts contained in the heavy ash.
 50. The method according to claim30, further comprising cooling the ashes carried out by said combustionfumes.
 51. The method according to claim 50, wherein said cooling iscarried out on said extractor.
 52. The method according to claim 30,further comprising directly feeding unburnt biomass on said extractor,upstream or in correspondence of said post-combustion portion.
 53. Themethod according to claim 52, further comprising feeding hot air to anunburnt biomass feeder, to perform a first drying of the unburnt biomassfeeder.
 54. The method according to claim 53, wherein said hot air isobtained by an air/combustion fumes heat exchange.
 55. The methodaccording to claim 30, further comprising providing dedicated crushingmeans, suitable for crushing biomass according to a preset maximum endgrain size.
 56. The method according to claim 55, further comprisingstoring the biomass into a same type of bunkers used for solid fuel andsupplying the biomass from said bunkers to said dedicated crushingmeans.
 57. The method according to claim 56, wherein said bunkers areassociated to burners of the combustion chamber arranged at topmostlevels.
 58. The method according to claim 55, further comprisingproviding, downstream of said dedicated crushing means, a screen forintercepting particles of a grain size greater than a predeterminedthreshold, and resending said particles to said dedicated crushingmeans.